The present study is aimed at the thermodynamic and statistical mechanical analyses of the association of local anesthetics with lipids, phospholipid membranes, protein macromolecules and nerve cell membrane fragments. At physiological pH, the clinically used aromatic amine local anesthetics exist in positively charged quaternary amines and in uncharged tertiary amines. It is generally assumed that the uncharged species are lipophilic and the charged species are hydrophilic. However, these molecules contain hydrophobic benzene ring at the other end of the structure, and both species are amphiphilic. Our study showed that the solubility of uncharged molecules in organic solvents decreased when the dielectric constants of the solvents were less than 10, indicating the propensity of these molecules to accumulate at the membrane/water inferface. We postulate that their actions may be exerted at the interface and both species are effective. Amphiphilicity rather than lipophilicity may be the key factor for their nerve blocking activities. The interfacial action can be measured by the surface tension method. Our statistical mechanical theory provides the means to separate the interaction energies into the anesthetic-lipid and the anesthetic-anesthetic interactions. The former energy represents the lipid-drug interaction, while the latter represents, the exclusion of the drug molecules from the structured bulk water. From the measurements of the equilibrium penetration of local anesthetics into the phospholipid monolayer, the above interaction energies will be obtained for the charged and uncharged molecules separately. By measuring the temperature dependence of the association of local anesthetics to model proteins, liposomes and cell membranes, the free energy of binding is separated into the enthalpy and entropy terms. By combining these observations, the thermodynamic basis of the mode of action of local anesthetics will be elucidated.